Systems and Methods for Non-Binary Decoding Biasing Control

ABSTRACT

Various embodiments of the present invention provide systems and methods for data processing. As an example, a data processing circuit is disclosed that includes a data detector circuit, a biasing circuit, and a data decoder circuit. The data detector circuit is operable to apply a data detection algorithm to a series of symbols to yield a detected output, and the detected output includes a series of soft decision data corresponding to non-binary symbols. The biasing circuit is operable apply a bias to each of the series of soft decision data to yield a series of biased soft decision data. The data decoder circuit is operable to apply a data decoding algorithm to the series of biased soft decision data corresponding to the non-binary symbols.

BACKGROUND OF THE INVENTION

The present inventions are related to systems and methods for storingdata, and more particularly to systems and methods for characterizing astorage device.

Manufacturing storage devices includes a process of operating a givendevice to determine whether it satisfies defined quality criteria. Asthe quality criteria may allow only a small number of failures, it maytake a long period of testing to determine whether the small number offailures occurs or not. This long period of time increases the cost ofcharacterizing the device.

Hence, for at least the aforementioned reasons, there exists a need inthe art for advanced systems and methods for characterizing a storagedevice.

BRIEF SUMMARY OF THE INVENTION

The present inventions are related to systems and methods for storingdata, and more particularly to systems and methods for characterizing astorage device.

Various embodiments of the present invention provide data processingcircuits. Such data processing circuits include a data detector circuit,a biasing circuit, and a data decoder circuit. The data detector circuitis operable to apply a data detection algorithm to a series of symbolsto yield a detected output, and the detected output includes a series ofsoft decision data corresponding to non-binary symbols. The biasingcircuit is operable apply a bias to each of the series of soft decisiondata to yield a series of biased soft decision data. The data decodercircuit is operable to apply a data decoding algorithm to the series ofbiased soft decision data corresponding to the non-binary symbols. Insome instances of the aforementioned embodiments, the data detectionalgorithm is a non-binary maximum a posteriori data detection algorithm.In other instances of the aforementioned embodiments, the data detectionalgorithm is a non-binary Viterbi data detection algorithm. In someembodiments of the present invention, the data decoding algorithm is anon-binary low density parity check decoding algorithm.

In some instances of the aforementioned embodiments, applying a bias toeach of the series of soft decision data includes: converting the seriesof soft decision data to a corresponding series of relative metrics;biasing a subset of the relative metrics to yield a series of biasedrelative metrics; and converting the series of biased relative metricsto a corresponding series of biased soft decision data. In some suchinstances, the series of soft decision data is a series of non-binarysoft decision data indicating a likelihood that respective symbol valuesare correct. In particular cases, the series of biased soft decisiondata is a series of biased non-binary soft decision data indicating alikelihood that respective symbol values are correct, and the series ofbiased non-binary soft decision data indicate a lower likelihood thancorresponding instances of the series of non-biased soft decision data.

In other instances of the aforementioned embodiments, biasing the subsetof the relative metrics to yield the series of biased relative metricsincludes multiplying each of the subset of the relative metrics by abias value. In some such instances, the circuit further includes a biasvalue memory that is operable to receive an address from the biasingcircuit and to return the bias value corresponding to the address. Inparticular instances, the address is derived from one of the series ofrelative metrics.

Other embodiments of the present invention provide methods forpredictably increasing an error rate of a data processing circuit. Suchmethods include: receiving a data set, wherein the data set includes aseries of symbols; applying a data detection algorithm by a datadetector circuit to a series of symbols to yield a detected output, andthe detected output includes a series of soft decision datacorresponding to non-binary symbols; biasing each of the series of softdecision data to yield a series of biased soft decision data; andapplying a data decoding algorithm by a data decoder circuit to theseries of biased soft decision data corresponding to the non-binarysymbols. In some such instances, biasing each of the series of softdecision data to yield the series of biased soft decision data includes:converting the series of soft decision data to a corresponding series ofrelative metrics; biasing a subset of the relative metrics to yield aseries of biased relative metrics; and converting the series of biasedrelative metrics to a corresponding series of biased soft decision data.

This summary provides only a general outline of some embodiments of theinvention. Many other objects, features, advantages and otherembodiments of the invention will become more fully apparent from thefollowing detailed description, the appended claims and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the various embodiments of the presentinvention may be realized by reference to the figures which aredescribed in remaining portions of the specification. In the figures,like reference numerals are used throughout several figures to refer tosimilar components. In some instances, a sub-label consisting of a lowercase letter is associated with a reference numeral to denote one ofmultiple similar components. When reference is made to a referencenumeral without specification to an existing sub-label, it is intendedto refer to all such multiple similar components.

FIG. 1 depicts a data processing circuit including selectable operationdegrading circuitry in accordance in some embodiments of the presentinvention;

FIG. 2 depicts a soft data biasing circuit that may be used in relationto the data processing circuits described above in relation to FIG. 1;

FIG. 3 depicts another data processing circuit including selectableoperation degrading circuitry in accordance in some embodiments of thepresent invention;

FIG. 4 is a flow diagram showing a method for selectable soft databiasing in accordance with some embodiments of the present invention;

FIG. 5 a is a histogram diagram showing distribution of soft dataoutputs for both a standard operation and a degraded operation;

FIG. 5 b is a histogram diagram showing distribution of normalized softdata outputs for both a standard operation and a degraded operation;

FIG. 6 shows a data transmission system including a receiver withperformance degradation control circuitry in accordance with one or moreembodiments of the present invention;

FIG. 7 shows a storage system including a read channel with performancedegradation control circuitry in accordance with one or more embodimentsof the present invention;

FIG. 8 is a flow diagram showing a method for determining degradationfactors or bias values in accordance with some embodiments of thepresent invention; and

FIG. 9 depicts a histogram based bias value calculation circuit inaccordance with various embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions are related to systems and methods for storingdata, and more particularly to systems and methods for characterizing astorage device.

FIG. 1 depicts a data processing circuit 100 including selectableoperation degrading circuitry in accordance in some embodiments of thepresent invention. Data processing circuit 100 includes an analog frontend circuit 110 that receives an analog input 105 and provides aprocessed analog signal 112. Analog front end circuit 110 may be anyanalog processing circuit known in the art. In one particular embodimentof the present invention, analog front end circuit 110 includes anamplifier circuit and a continuous time filter (i.e., an analog filter)that operate to amplify and noise filter the received input to yieldprocessed analog signal 112. Analog signal 105 may be derived from avariety of sources including, but not limited to, a storage medium or atransfer medium. Based upon the disclosure provided herein, one ofordinary skill in the art will recognize a variety of sources from whichanalog input signal 105 may be derived.

Processed analog signal 112 is provided to an analog to digitalconverter circuit 114 that samples processed analog signal 112 to yielda corresponding series of digital samples 116. Analog to digitalconverter circuit 114 may be any circuit known in the art that iscapable of converting a continuous time signal to a series of digitalvalues. Digital samples 116 are provided to an equalizer circuit 120that may be implemented as a digital finite impulse response circuit asare known in the art. Equalizer circuit 120 performs an equalization ondigital samples 116 to yield an equalized output 125. Based upon thedisclosure provided herein, one of ordinary skill in the art willrecognize a variety of equalizer circuits that may be used in relationto different embodiments of the present invention.

Equalized output 125 is provided to a data detector circuit 135 and to abuffer circuit 130. Data detector circuit 135 applies a data detectionalgorithm to equalized output 125 to yield a detected output 137. Datadetector circuit 135 may be any data detector circuit known in the artincluding, but not limited to, a Viterbi algorithm data detector circuitor a maximum a posteriori data detector circuit. In some cases, detectedoutput 137 includes both hard decision data and soft decision data. Asused herein, the phrase “hard decision data” refers to a data outputthat indicates a certain logic level for a given bit period or bitperiods. For example, in a binary system, the logic level may be a ‘1’or a ‘0’. As another example, in a two bit binary system, the logiclevel may be ‘00’, ‘01’, ‘10’ or ‘11’. Based upon the disclosureprovided herein, one of ordinary skill in the art will recognize avariety of hard decision data that may be produced by a data detectorcircuit. As used herein, the phrase “soft decision data” refers to adata output that indicates a likelihood that data detector circuit 135correctly detected data for the bit period or bit periods. In oneparticular embodiment of the present invention, soft decision dataranges from negative fifteen (−15) to positive fifteen (+15) with −15indicating that the decision is highly unlikely to be correct and +15indicating that the decision is highly likely to be correct. Theincreasing values from −14 to +14 correspond to a corresponding increasein the likelihood that the decision was correct. Based upon thedisclosure provided herein, one of ordinary skill in the art willrecognize a variety of values and ranges that may be used to representthe soft decision data.

In a non-binary data processing system, groups of symbols are processedtogether. For example, in a two bit, non-binary data processing systemgroups of two bit symbols are processed together with the groups of twobit symbols being capable of representing four possible outputs (i.e.,00, 01, 10, 11). In such systems, metrics (i.e., Euclidean distances)are calculated for each of the possible symbol values. In the case oftwo bit symbols, four metrics are calculated. A large metric valueindicates that the corresponding symbol value is unlikely, and a smallmetric indicates a higher degree of likelihood. A symbol is in errorwhen the correctly selected symbol value does not correspond to thesmallest of the metrics. In contrast, a symbol is determined to becorrect when the correctly selected symbol value corresponds to thesmallest of the metrics. The later discussed decoding algorithmprocesses the multiple metrics to correct any errors. Detected output137 includes the metrics for each of the possible values for the symbolvalues as soft data decisions, and may or may not include the symbolvalue corresponding to the smallest metric.

Detected output 137 is provided to both a soft data biasing circuit 140and a selector circuit 150. Soft data biasing circuit 140 is operable tobias the soft decision data included in detected output 137 to yield amodified detected output 142. In particular, the soft decision data fromdetected output 137 is converted from non-binary metrics to relativemetrics, biased by applying noise biasing to the relative metrics toyield biased relative metrics, and the biased relative metrics convertedback into non-binary metrics (i.e., updated binary metrics).

Converting from the non-binary metrics to relative metrics includescalculating relative differences between the received non-binary metricssuch that a zero value is assigned to the metric corresponding to themost likely symbol value, and non-negative metrics are assigned to allother possible symbol values. Using a two-bit, non-binary dataprocessing system as an example where there are four possible symbolvalues (i.e., 00, 01. 10, 11) and corresponding non-binary metrics(i.e., M₀₀, M₀₁, M₁₀, and M₁₁) included in detected output 137, relativemetrics (RM₀₀, RM₀₁, RM₁₀, and RM₁₁) are calculated in accordance withthe following equations:

RM₀₀ =M ₀₀−Reference Metric,

RM₀₁ =M ₀₁−Reference Metric,

RM₁₀ =M ₁₀−Reference Metric, and

RM₁₁ =M ₁₁−Reference Metric,

where the Reference Metric is the metric value corresponding to the truesymbol value (i.e., a test pattern) that was written. The metric valuecorresponding to the true symbol value may be regenerated from memory orusing another approach such as accessing the reference metric from apre-programmed memory.

As will be appreciated, the relative metric corresponding to the correctsymbol value is zero where it was properly selected. If the datadetection is correct, then all metrics would be non-negative.Alternatively, if detection is not correct, then the symbol with theminimum metric will be different from the correct symbol. In this caseof an incorrect detection, the relative metric for the detector haddecision symbol is negative.

Soft data biasing circuit 140 biases the non-zero relative metrics toyield biased relative metrics. This may be done in a way that distortsdetected output 137 such that the errors resulting from the distortionis predictably related to the number of errors that would result whereno distortion is applied. By predictably degrading the operation of dataprocessing circuit 100, the time required to generate a sufficientnumber of errors to characterize a system in which data processingcircuit 100 is deployed is decreased, and because the number of errorscorresponding from the degraded operation is predictably related to anactual number of errors, the characterization represents the actualoperation of the system. In some cases, all of the non-zero relativemetrics are replaced by biased relative metrics 167 accessed from abiasing look up table 165. Biased relative metrics 167 corresponds tothe corresponding calculated relative metrics that are provided tobiasing look up table 165 as an address 169. In turn, soft data biasingcircuit 140 replaces the respective relative metrics using the returnedbiased relative metrics.

It has been determined that the errors are mostly related to the mostlikely symbol (with a small relative metric value), and the biasingfunction for the other less likely symbol values is less relevant toerror correction performance. Because of this, the same bias value maybe applied to all of the non-zero relative metrics in an effort to savehardware complexity. Thus, for example, a single relative metric isprovided to biasing look up table 165 as an address 169 that returns asingle biased relative metric 167. This biased relative metric 167 isused in place of all of the non-zero relative metrics. In other cases,the bias values may be selected based upon the correctly selected symbolvalue (i.e., the lowest relative metric value) as in a magnetic basedstorage device there is intrinsically more noise with transitioningsymbols (e.g., ‘01’ and ‘10’) compared with non-transitioning symbols(e.g., ‘11’ and ‘00’) due to the nature of magnetic recording physics.

Converting the biased relative metrics back into non-binary metrics issubstantially the inverse of the previously described conversion fromthe non-binary metrics into the relative metrics. Using the sametwo-bit, non-binary data processing system as an example, updatednon-binary metrics (i.e., UNBM₀₀, UNBM₀₁, UBM₁₀, and UNBM₁₁) arecalculated from the biased relative metrics (i.e., BRM₀₀, BRM₀₁, BRM₁₀,and BRM₁₁) in accordance with the following equations:

UNBM₀₀=BRM₀₀−Minimum Relative Metric,

UNBM₀₁=BRM₀₁−Minimum Relative Metric,

UNBM₁₀=BRM₁₀−Minimum Relative Metric, and

UNBM₁₁=BRM₁₁−Minimum Relative Metric,

where the Minimum Relative Metric is the biased relative metric (BRM₀₀,BRM₀₁, BRM₁₀, and BRM₁₁) that has the lowest value. The updatednon-biased metrics are provided as part of modified detected output 142.It should be noted that while the example of soft biasing circuit 140 isa two-bit example that the same approach may be expanded for applicationto non-binary data processing systems operating on symbols includingthree or more bits per symbol.

In addition, soft data biasing circuit 140 provides soft decision datafrom detected output 137 and soft decision data from modified detectedoutput 142 as a soft decision output 144 to a histogram based look uptable calculation circuit 180. Histogram based look up table calculationcircuit 180 histograms the instances of the received soft decisionoutput 144 corresponding to multiple instances of detected output 137.Histograms corresponding to two different instances of detected output137 (i.e., a first series of detected outputs corresponding to a firstcodeword, and a second series of detected outputs corresponding to asecond codeword) each exhibiting a different error rate are used tocalculate bias values 182 corresponding to the degradation between thetwo instances of detected output 137.

Turning to FIG. 5 a, a graphical depiction 500 shows an example ofhistogram A (curve 501) and histogram B (curve 503). In particular,histogram A (curve 501) shows the normalized number of soft decisionoutput 144 from detected output 137 that have a log-likelihood ratio of−15 (highly unlikely to be correct) is about 10⁻⁵. Each value of thelog-likelihood ratio from −14 to 15 increases from about 10⁻⁵ to justless than 10°. Similarly, histogram B (curve 503) shows the normalizednumber of soft decision output 144 from detected output 137 that have alog-likelihood ratio of −15 (highly unlikely to be correct) is about10⁻⁴. Each value of the log-likelihood ratio from −14 to 15 increasesfrom about 10⁻⁴ to just less than 10°. The error rate of the data setcorresponding to histogram A (curve 501) is less than that of the dataset corresponding to histogram B (curve 503) as evidenced by anincreased likelihood of low value soft decision outputs 144 from thedata set. Turning to FIG. 5 b, a graphical depiction 550 shows anexample of a cumulative mass function corresponding to histogram A(curve 551) and a cumulative mass function of histogram B (curve 553).

Using the example histogram A and histogram B, the process ofcalculating bias values 182 includes: (1) histogram the data received assoft decision outputs 144 to yield histogram A and histogram B, (2)normalize histogram A and histogram B to yield data similar to curve 501and curve 503, (3) compute a normalized cumulative mass function for thereceived histogram data, and (4) compute bias values 182 using thecalculated cumulative mass function. Normalizing histogram A andhistogram B, computing the normalized mass function, and computing biasvalues 182 may be done in accordance with the following pseudocode:

/* (2) normalize histogram A and histogram B */ For (i=1 to Length ofHistogram A) { normalized histogram A[i] = (histogram A[i])/(sum of allelements of histogram A); normalized histogram B[i] = (histogramB)[i]/(sum of all elements of histogram B) } /*(3) compute a normalizedcumulative mass function for the received histogram data*/ cumulativemass function A (CMFA[1]) = normalized histogram A[1]; cumulative massfunction B (CMFB[1]) = normalized histogram B[1];  For (i=2 to Length ofHistogram A) { CMFA[i] = CMFA[i−1]+ normalized histogram A[i]; CMFB[i] =CMFB[i−1]+ normalized histogram B[i] } /*(4) compute bias values 182using the calculated cumulative mass function */ Length of Biasing LookUp Table 165 (LUT Length) = Length of Histogram A; Bias Value 182 (LUT[]) = Array[LUT Length]; k = 1; For (i=1 to LUT Length) { if (i==1) {LUT[i] = 1 } Else { LUT[i] = LUT[i−1] } For (j=LUT[i] to Length ofHistogram A) { If (abs_value(CMFA[i]− CMFA[LUT[i]])> abs_value(CMFA[i]−CMFA[j]) { LUT[i]=j } } While (k<=LUT[i]) { Updated Cumulative MassFunction (UCMFA[ ]) = CMFA[i]; k = k+1 } }

The aforementioned pseudocode assumes that the number of instances inhistogram A (i.e., length of histogram A) is the same as that ofhistogram B. Returning to FIG. 1, at the end of processing, LUT[i] thatcorresponds to bias values 182 represent the biased relative metric usedto replace a corresponding relative metric value such that histogram Ais replaced by histogram B. These values are stored to biasing look uptable 165. As such, when a value corresponding to one of the values inhistogram A is selected by an address 169 (e.g., a log likelihood ratiobetween −15 and 15 corresponding to one of the elements of histogram A),a corresponding biased relative metric 167 is provided by biasing lookup table 165 to soft data biasing circuit 140. In turn, soft databiasing circuit 140 replaces the relative metric with the received biasrelative metric. These biased relative metrics are then transformed intothe aforementioned updated non-binary metrics.

The calculated bias values 182 are stored to a bias look up table 165.As previously mentioned, non-zero relative metrics are calculated and anaddress 169 corresponding to the non-zero relative metrics is providedto biasing look up table 165. In response, biasing look up table 165provides bias values 167 that may be used to degrade the circuitoperation to exhibit a higher error rate difference corresponding tothat of the two histograms used to calculate bias values 182.

Selector circuit 150 selects between detected output 137 and modifieddetected output 142 to provide as a selected output 152. The selectionbetween detected output 137 and modified detected output 142 is madebased upon a selector input 172 that is controlled by a biasing controlcircuit 170. In some embodiments of the present invention, biasingcontrol circuit 170 is a programmable register. When selector input 172is asserted as a logic ‘1’, modified detected output 142 is provided asselected output 152. Alternatively, when selector input 172 is assertedas a logic ‘0’, detected output 137 is provided as selected output 152.

Selected output 152 is provided to a data decoder circuit 155. Datadecoder circuit 155 applies a data decoding algorithm to selected output152 to yield a decoded output 157. Data decoder circuit 155 may be anydata decoder circuit known in the art. In one embodiment of the presentinvention, data decoder circuit 155 is a non-binary low density paritycheck decoder circuit. Where decoded output 157 fails to converge (i.e.,includes errors or differences from an originally written data set),data detector circuit 135 may reprocess the data set used to derivedecoded output 157 that is retrieved from buffer circuit 130 as buffereddata 132. The reprocessing is done using decoded output 157 as a guide.The combination of applying both the data detection algorithm and thedata decoding algorithm is referred to as a global iteration. Theprocess of deriving the original data set may include many globaliterations.

Turning to FIG. 2, one implementation of a soft data biasing circuit 200is shown that may be used in place of soft data biasing circuit 140 ofFIG. 1. Soft data biasing circuit 200 includes a non-binary metrics torelative metrics conversion circuit 205. The non-binary metricscorresponds to the soft decision data derived from detected output 137.Converting from the non-binary metrics to relative metrics includescalculating relative differences between the received non-binary metricssuch that a zero value is assigned to the metric corresponding to themost likely symbol value, and to non-negative metrics are assigned toall other possible symbol values. Using a two-bit, non-binary dataprocessing system as an example where there are four possible symbolvalues (i.e., 00, 01. 10, 11) and corresponding non-binary metrics(i.e., M₀₀, M₀₁, M₁₀, and M₁₁) included in detected output 137, relativemetrics (RM₀₀, RM₀₁, RM₁₀, and RM₁₁) are calculated in accordance withthe following equations:

RM₀₀ =M ₀₀−Reference Metric,

RM₀₁ =M ₀₁−Reference Metric,

RM₁₀ =M ₁₀−Reference Metric, and

RM₁₁ =M ₁₁−Reference Metric,

where the Reference Metric is the metric value corresponding to the truesymbol value (i.e., a test pattern) that was written. The metric valuecorresponding to the true symbol value may be regenerated from memory orusing another approach such as accessing the reference metric from apre-programmed memory.

The aforementioned Lowest Metric is provided by conversion circuit 205as an address 169 to biasing look up table 165. In return, biasing lookup table 165 provides a corresponding biased relative metric 167. Biasedrelative metric 167 is provided to a biased relative metrics replacementcircuit 220. Biased relative metrics replacement circuit 220 replacesnon-zero relative metrics (three of RM₀₀, RM₀₁, RM₁₀, and RM₁₁) withrespective values received as biased relative metrics (three of BRM₀₀,BRM₀₁, BRM₁₀, and BRM₁₁). In some cases, biased relative metric 167 is asingle biased relative metric that is used to replace the three non-zerorelative metrics. In other cases, biased relative metric 167 is threevalues used to replace corresponding to the three non-zero relativemetrics.

The biased relative metrics (BRM₀₀, BRM₀₁, BRM₁₀, BRM₁₁) are provided toa biased relative metrics to updated non-binary metrics conversioncircuit 230. Conversion circuit 230 performs substantially the inverseconversion of that performed by conversion circuit 205. Using the sametwo-bit, non-binary data processing system as an example, updatednon-binary metrics (i.e., UNBM₀₀, UNBM₀₁, UBM₁₀, and UNBM₁₁) arecalculated from the biased relative metrics (i.e., BRM₀₀, BRM₀₁, BRM₁₀,and BRM₁₁) in accordance with the following equations:

UNBM₀₀=BRM₀₀−Minimum Relative Metric,

UNBM₀₁=BRM₀₁−Minimum Relative Metric,

UNBM₁₀=BRM₁₀−Minimum Relative Metric, and

UNBM₁₁=BRM₁₁−Minimum Relative Metric,

where the Minimum Relative Metric is the biased relative metric (BRM₀₀,BRM₀₁, BRM₁₀, and BRM₁₁) that has the lowest value. The updatednon-biased metrics are provided as part of modified detected output 142.It should be noted that while the example of soft biasing circuit 200 isa two-bit example that the same approach may be expanded for applicationto non-binary data processing systems operating on symbols includingthree or more bits per symbol.

Turning to FIG. 3, another data processing circuit 300 includingselectable operation degrading circuitry is shown in accordance in someembodiments of the present invention. The fundamental difference betweendata processing circuit 100 and data processing circuit 300 is that dataprocessing circuit 300 uses a fixed or programmable biased valuecompared to the adaptive bias value generated by data processing circuit100. In some cases, the fixed or programmable bias value used in dataprocessing circuit may be calculated using the same histogram basedapproach discussed above in relation to FIG. 1, but is not adaptivelyupdated. In other cases, the fixed or programmable bias value iscalculated or selected using another approach.

Data processing circuit 300 includes an analog front end circuit 310that receives an analog input 305 and provides a processed analog signal312. Analog front end circuit 310 may be any analog processing circuitknown in the art. In one particular embodiment of the present invention,analog front end circuit 310 includes an amplifier circuit and acontinuous time filter (i.e., an analog filter) that operate to amplifyand noise filter the received input to yield processed analog signal312. Analog signal 305 may be derived from a variety of sourcesincluding, but not limited to, a storage medium or a transfer medium.Based upon the disclosure provided herein, one of ordinary skill in theart will recognize a variety of sources from which analog input signal305 may be derived.

Processed analog signal 312 is provided to an analog to digitalconverter circuit 314 that samples processed analog signal 312 to yielda corresponding series of digital samples 316. Analog to digitalconverter circuit 314 may be any circuit known in the art that iscapable of converting a continuous time signal to a series of digitalvalues. Digital samples 316 are provided to an equalizer circuit 320that may be implemented as a digital finite impulse response circuit asare known in the art. Equalizer circuit 320 performs an equalization ondigital samples 316 to yield an equalized output 325. Based upon thedisclosure provided herein, one of ordinary skill in the art willrecognize a variety of equalizer circuits that may be used in relationto different embodiments of the present invention.

Equalized output 325 is provided to a data detector circuit 335 and to abuffer circuit 330. Data detector circuit 335 applies a data detectionalgorithm to equalized output 325 to yield a detected output 337. Datadetector circuit 335 may be any data detector circuit known in the artincluding, but not limited to, a Viterbi algorithm data detector circuitor a maximum a posteriori data detector circuit. In some cases, detectedoutput 337 includes both hard decision data and soft decision data. Asused herein, the phrase “hard decision data” refers to a data outputthat indicates a certain logic level for a given bit period or bitperiods. For example, in a binary system, the logic level may be a ‘1’or a ‘0’. As another example, in a two bit binary system, the logiclevel may be ‘00’, ‘01’, ‘10’ or ‘11’. Based upon the disclosureprovided herein, one of ordinary skill in the art will recognize avariety of hard decision data that may be produced by a data detectorcircuit. As used herein, the phrase “soft decision data” refers to adata output that indicates a likelihood that data detector circuit 335correctly detected data for the bit period or bit periods. In oneparticular embodiment of the present invention, soft decision dataranges from negative fifteen (−15) to positive fifteen (+15) with −15indicating that the decision is highly unlikely to be correct and +15indicating that the decision is highly likely to be correct. Theincreasing values from −14 to +14 correspond to a corresponding increasein the likelihood that the decision was correct. Based upon thedisclosure provided herein, one of ordinary skill in the art willrecognize a variety of values and ranges that may be used to representthe soft decision data.

In a non-binary data processing system, groups of symbols are processedtogether. For example, in a two bit, non-binary data processing systemgroups of two bit symbols are processed together with the groups of twobit symbols being capable of representing four possible outputs (i.e.,00, 01, 10, 11). In such systems, metrics (i.e., Euclidean distances)are calculated for each of the possible symbol values. In the case oftwo bit symbols, four metrics are calculated. A large metric valueindicates that the corresponding symbol value is unlikely, and a smallmetric indicates a higher degree of likelihood. A symbol is in errorwhen the correctly selected symbol value does not correspond to thesmallest of the metrics. In contrast, a symbol is determined to becorrect when the correctly selected symbol value corresponds to thesmallest of the metrics. The later discussed decoding algorithmprocesses the multiple metrics to correct any errors. Detected output337 includes the metrics for each of the possible values for the symbolvalues as soft data decisions, and may or may not include the symbolvalue corresponding to the smallest metric.

Detected output 337 is provided to both a soft data biasing circuit 340and a selector circuit 350. Soft data biasing circuit 340 is operable tobias the soft decision data included in detected output 337 to yield amodified detected output 342. In particular, the soft decision data fromdetected output 337 is converted from non-binary metrics to relativemetrics, biased by applying noise biasing to the relative metrics toyield biased relative metrics, and the biased relative metrics convertedback into non-binary metrics (i.e., updated binary metrics).

Converting from the non-binary metrics to relative metrics includescalculating relative differences between the received non-binary metricssuch that a zero value is assigned to the metric corresponding to themost likely symbol value, and to non-negative metrics are assigned toall other possible symbol values. Using a two-bit, non-binary dataprocessing system as an example where there are four possible symbolvalues (i.e., 00, 01. 10, 11) and corresponding non-binary metrics(i.e., M₀₀, M₀₁, M₁₀, and M₁₁) included in detected output 137, relativemetrics (RM₀₀, RM₀₁, RM₁₀, and RM₁₁) are calculated in accordance withthe following equations:

RM₀₀=M₀₀−Reference Metric,

RM₀₁=M₀₁−Reference Metric,

RM₁₀=M₁₀−Reference, and

RM₁₁=M₁₁−Reference Metric,

where the Reference Metric is the metric value corresponding to the truesymbol value (i.e., a test pattern) that was written. The metric valuecorresponding to the true symbol value may be regenerated from memory orusing another approach such as accessing the reference metric from apre-programmed memory.

As will be appreciated, the relative metric corresponding to the correctsymbol value is zero where it was properly selected. Thus, for each ofthe incorrect symbol values (i.e., the symbol values that do notcorrespond to the Lowest Metric), if the corresponding relative metricis positive, application of the data detection algorithm is correctlyselecting the appropriate symbol value. Conversely, if any of therelative metrics is negative, application of the data detectionalgorithm is not correctly selecting the appropriate symbol value.

Soft data biasing circuit 340 biases the non-zero relative metrics toyield biased relative metrics. This may be done in a way that distortsdetected output 337 such that the errors resulting from the distortionis predictably related to the number of errors that would result whereno distortion is applied. By predictably degrading the operation of dataprocessing circuit 300, the time required to generate a sufficientnumber of errors to characterize a system in which data processingcircuit 300 is deployed is decreased, and because the number of errorscorresponding from the degraded operation is predictably related to anactual number of errors, the characterization represents the actualoperation of the system. In some cases, all of the non-zero relativemetrics are multiplied by the same fixed bias value. In other cases, allof the non-zero relative metrics are multiplied by the same bias valuethat is selected based upon one or a combination of the non-zerorelative metrics. In yet other cases, each of the non-zero relativemetrics is multiplied by a respective bias value selected based on thecorresponding relative metric. In cases where individual bias values areused, the bias value may be selected based upon both the value of therelative metric and the proximity to zero (i.e., whether it is theclosest to zero, the second closest to zero, or the third closest tozero).

It has been determined that the errors are mostly related to the mostlikely symbol (with a small relative metric value), and the biasingfunction for the other less likely symbol values is less relevant toerror correction performance. Because of this, the same bias value maybe applied to all of the non-zero relative metrics in an effort to savehardware complexity. In other cases, the bias values may be selectedbased upon the correctly selected symbol value (i.e., the lowestrelative metric value) as in a magnetic based storage device there isintrinsically more noise with transitioning symbols (e.g., ‘01’ and‘10’) compared with non-transitioning symbols (e.g., ‘11’ and ‘00’) dueto the nature of magnetic recording physics.

In some embodiments of the present invention, the biasing is done bymultiplying the relative metrics calculated by soft data biasing circuit340 by a bias value 367 to yield respective biased relative metrics(BRM). Bias value 367 is provided by a fixed bias values register 365that may be either programmable or hardwired depending upon theparticular implementation. Soft data biasing circuit 340 then convertsthe biased relative metrics back into non-binary metrics using a processthat is substantially the inverse of the previously described conversionfrom the non-binary metrics into the relative metrics. Using the sametwo-bit, non-binary data processing system as an example, updatednon-binary metrics (i.e., UNBM₀₀, UNBM₀₁, UBM₁₀, and UNBM₁₁) arecalculated from the biased relative metrics (i.e., BRM₀₀, BRM₀₁, BRM₁₀,and BRM₁₁) in accordance with the following equations:

UNBM₀₀=BRM₀₀−Minimum Relative Metric,

UNBM₀₁=BRM₀₁−Minimum Relative Metric,

UNBM₁₀=BRM₁₀−Minimum Relative Metric, and

UNBM₁₁=BRM₁₁−Minimum Relative Metric,

where the Minimum Relative Metric is the biased relative metric (BRM₀₀,BRM₀₁, BRM₁₀, and BRM₁₁) that has the lowest value. The updatednon-biased metrics are provided as part of modified detected output 342.It should be noted that while the example of soft biasing circuit 340 isa two-bit example that the same approach may be expanded for applicationto non-binary data processing systems operating on symbols includingthree or more bits per symbol.

Selector circuit 350 selects between detected output 337 and modifieddetected output 142 to provide as a selected output 352. The selectionbetween detected output 337 and modified detected output 342 is madebased upon a selector input 372 that is controlled by a biasing controlcircuit 370. In some embodiments of the present invention, biasingcontrol circuit 370 is a programmable register. When selector input 372is asserted as a logic ‘1’, modified detected output 342 is provided asselected output 352. Alternatively, when selector input 372 is assertedas a logic ‘0’, detected output 337 is provided as selected output 352.

Selected output 352 is provided to a data decoder circuit 355. Datadecoder circuit 355 applies a data decoding algorithm to selected output352 to yield a decoded output 357. Data decoder circuit 355 may be anydata decoder circuit known in the art. In one embodiment of the presentinvention, data decoder circuit 355 is a non-binary low density paritycheck decoder circuit. Where decoded output 357 fails to converge (i.e.,includes errors or differences from an originally written data set),data detector circuit 335 may reprocess the data set used to derivedecoded output 357 that is retrieved from buffer circuit 330 as buffereddata 332. The reprocessing is done using decoded output 357 as a guide.The combination of applying both the data detection algorithm and thedata decoding algorithm is referred to as a global iteration. Theprocess of deriving the original data set may include many globaliterations.

Turning to FIG. 4, a flow diagram 400 shows a method for selectable softdata biasing in accordance with some embodiments of the presentinvention. Following flow diagram 400, an analog input is received(block 405). The analog input may correspond to information maintainedon a storage medium. The analog input is converted to a series ofdigital samples (block 410). The analog to digital conversion processmay be any process known in the art for converting an analog signal tocorresponding digital samples. A data equalization process is thenapplied to the digital samples to yield an equalized output (block 415).In some embodiments of the present invention, the equalization processis performed by one or more digital finite impulse filters. Based uponthe disclosure provided herein, one of ordinary skill in the art willrecognize a variety of equalization methods that may be used in relationto different embodiments of the present invention.

Data detection is performed on the equalized output (block 420). Thedata processing may include, but is not limited to, applying a datadetection algorithm to the equalized output to yield a detected output.In one embodiment, the data detection process is a Viterbi algorithmdetection process or a maximum a posteriori detection process as areknown in the art. The detected output may include both hard decisiondata and soft decision data. Based upon the disclosure provided herein,one of ordinary skill in the art will recognize a variety of datadetection processes and/or algorithms that can be used in relation todifferent embodiments of the present invention.

It is determined whether soft data marginalization is selected (block425). Such soft data marginalization may be selected to predictablyincrease the number of errors resulting from data processing. Bypredictably increasing the number of errors resulting from dataprocessing, the time required to generate a sufficient number of errorsto characterize a system in which the process is deployed is decreased.Because the number of errors corresponding from the marginalizedoperation is predictably related to an actual number of errors, thecharacterization represents the actual operation of the system.

Where soft data marginalization is selected (block 425), the non-binarymetrics included as the soft decision data of the detected output areconverted to relative metrics (block 430). Converting from thenon-binary metrics to relative metrics includes calculating relativedifferences between the received non-binary metrics such that a zerovalue is assigned to the metric corresponding to the most likely symbolvalue, and to non-negative metrics are assigned to all other possiblesymbol values. Using a two-bit, non-binary data processing system as anexample where there are four possible symbol values (i.e., 00, 01. 10,11) and corresponding non-binary metrics (i.e., M₀₀, M₀₁, M₁₀, and M₁₁)included in detected output 137, relative metrics (RM₀₀, RM₀₁, RM₁₀, andRM₁₁) are calculated in accordance with the following equations:

RM₀₀ =M ₀₀−Reference Metric,

RM₀₁ =M ₀₁−Reference Metric,

RM₁₀ =M ₁₀−Reference Metric, and

RM₁₁ =M ₁₁−Reference Metric,

where the Reference Metric is the metric value corresponding to the truesymbol value (i.e., a test pattern) that was written. The metric valuecorresponding to the true symbol value may be regenerated from memory orusing another approach such as accessing the reference metric from apre-programmed memory.

The relative metrics are then biased to yield biased relative metrics(BRM₀₀, BRM₀₁, BRM₁₀, and BRM₁₁) (block 435). This may be done, forexample, by accessing a biasing look up table based upon one of thenon-zero relative metrics as discussed above in relation to FIG. 1. Inother cases, biasing may be done by multiplying each of the relativemetrics by a bias value in accordance with the following equations:

BRM₀₀=(BiasValue)*(RM₀₀),

BRM₀₁=(BiasValue)*(RM₀₁),

BRM₁₀=(BiasValue)*(RM₁₀), and

BRM₁₁=(BiasValue)*(RM₁₁).

The bias value may be a fixed or programmable value similar to thatdiscussed above in relation to FIG. 3.

It should be noticed that other approaches may be used to apply thebias. For example, in other embodiments of the present invention,biasing may be done by subtracting the bias value from the correspondingrelative metric in accordance with the following equations:

BRM₀₀=RM₀₀−BiasValue,

BRM₀₁=RM₀₁−BiasValue,

BRM₁₀=RM₁₀−BiasValue, and

BRM₁₁=RM₁₁−BiasValue.

Based upon the disclosure provided herein, one of ordinary skill in theart will recognize other approaches for applying bias that may be usedin relation to different embodiments of the present invention.

The biased relative metrics are then converted to updated non-binarymetrics (UNBM₀₀, UNBM₀₁, UBM₁₀, and UNBM₁₁) (block 440). Using the sametwo-bit, non-binary data processing system as an example, updatednon-binary metrics (i.e., UNBM₀₀, UNBM₀₁, UBM₁₀, and UNBM₁₁) arecalculated from the biased relative metrics (i.e., BRM₀₀, BRM₀₁, BRM₁₀,and BRM₁₁) in accordance with the following equations:

UNBM₀₀=BRM₀₀−Minimum Relative Metric,

UNBM₀₁=BRM₀₁−Minimum Relative Metric,

UNBM₁₀=BRM₁₀−Minimum Relative Metric, and

UNBM₁₁=BRM₁₁−Minimum Relative Metric,

where the Minimum Relative Metric is the biased relative metric (BRM₀₀,BRM₀₁, BRM₁₀, and BRM₁₁) that has the lowest value. The updatednon-biased metrics are provided as part of a modified detected output(block 445). It should be noted that while the process of flow diagram400 is explained using a two bit symbol example that the same approachmay be expanded for application to non-binary data processing systemsoperating on symbols including three or more bits per symbol.

Where either soft data marginalization is not selected (block 425) orthe marginalization process has completed (blocks 430-445), standardprocessing of the detected output (block 420) is performed. Inparticular a data decoding algorithm is applied to the detected outputto yield a decoded output (block 450). The decoding algorithm may be,for example, a non-binary low density parity check algorithm as areknown in the art. Based upon the disclosure provided herein, one ofordinary skill in the art will recognize a variety of decodingalgorithms that may be applied in relation to different embodiments ofthe present invention. It is determined whether the decoded outputconverged (i.e., resulted in a correct identification of the originallywritten data) (block 455). Where the decoded output converged (block455), the decoded output is provided as a data output (block 460).Alternatively, where the decoded output failed to converge (block 455),a subsequent data detection process is applied using the decoded outputas a guide (block 420).

Turning to FIG. 6, a data transmission system 600 including a receiver620 having noise injection circuitry is shown in accordance with someembodiments of the present invention. Data transmission system 600includes a transmitter 610 that is operable to transmit encodedinformation via a transfer medium 630 as is known in the art. Theencoded data is received from transfer medium 630 by receiver 620.Receiver 620 incorporates performance degradation control circuitry thatmay be implemented similar to that discussed above in relation to FIG.1, FIG. 2, or FIG. 3 and/or operate similar to that described above inrelation to FIG. 4.

FIG. 7 shows a storage system 700 including a read channel circuit 710with performance degradation control circuitry in accordance with someembodiments of the present invention. Storage system 700 may be, forexample, a hard disk drive. Storage system 700 also includes apreamplifier 770, an interface controller 720, a hard disk controller766, a motor controller 768, a spindle motor 772, a disk platter 778,and a read/write head assembly 776. Interface controller 720 controlsaddressing and timing of data to/from disk platter 778. The data on diskplatter 778 consists of groups of magnetic signals that may be detectedby read/write head assembly 776 when the assembly is properly positionedover disk platter 778. In one embodiment, disk platter 778 includesmagnetic signals recorded in accordance with either a longitudinal or aperpendicular recording scheme.

In a typical read operation, read/write head assembly 776 is accuratelypositioned by motor controller 768 over a desired data track on diskplatter 778. Motor controller 768 both positions read/write headassembly 776 in relation to disk platter 778 and drives spindle motor772 by moving read/write head assembly to the proper data track on diskplatter 778 under the direction of hard disk controller 766. Spindlemotor 772 spins disk platter 778 at a determined spin rate (RPMs). Onceread/write head assembly 778 is positioned adjacent the proper datatrack, magnetic signals representing data on disk platter 778 are sensedby read/write head assembly 776 as disk platter 778 is rotated byspindle motor 772. The sensed magnetic signals are provided as acontinuous, minute analog signal representative of the magnetic data ondisk platter 778. This minute analog signal is transferred fromread/write head assembly 776 to read channel circuit 710 viapreamplifier 770. Preamplifier 770 is operable to amplify the minuteanalog signals accessed from disk platter 778. In turn, read channelcircuit 710 decodes and digitizes the received analog signal to recreatethe information originally written to disk platter 778. This data isprovided as read data 703 to a receiving circuit. A write operation issubstantially the opposite of the preceding read operation with writedata 701 being provided to read channel circuit 710. This data is thenencoded and written to disk platter 778.

During one or more set up periods, the included performance degradationcontrol circuitry degrades circuit operation to incur a number ofotherwise unexpected errors. During this process, soft data is modifiedto negatively impact circuit performance in a controlled way. As justone of many advantages, the ability to controllably induce errors allowsfor faster device characterization than is possible where a normal errorrate occurs. Such performance degradation may be done duringmanufacture, and/or may be done once storage device 700 is deployed. Theperformance degradation control circuitry may be implemented similar tothat discussed above in relation to FIG. 1, FIG. 2 or FIG. 3 and/oroperate similar to that described above in relation to FIG. 4.

It should be noted that storage system 700 may be integrated into alarger storage system such as, for example, a RAID (redundant array ofinexpensive disks or redundant array of independent disks) based storagesystem. It should also be noted that various functions or blocks ofstorage system 700 may be implemented in either software or firmware,while other functions or blocks are implemented in hardware.

Turning to FIG. 8, a flow diagram 800 shows a method for determiningdegradation factors or bias values in accordance with some embodimentsof the present invention. Following flow diagram 800, relative metricsare received from block 430 of FIG. 4. These relative metrics areconverted soft decision data indicating a likelihood that a given symbolvalue would be correct. A histogram of relative metrics corresponding toa first condition is calculated (block 810). The first condition is afirst data set (i.e., series of values) that when processed yield afirst error rate. The histogram corresponding to the first condition(block 810) is normalized (block 815). This normalization may be done inaccordance with the following pseudocode:

/* normalize histogram A */ For (i=1 to Length of Histogram A) {normalized histogram A[i] = (histogram A[i])/(sum of all elements ofhistogram A) }Histogram A corresponds to the histogram of the first condition. Thecumulative mass of the histogram of the first condition is calculated(block 817). This calculation may be done in accordance with thefollowing pseudocode:

/*compute a normalized cumulative mass function for the receivedhistogram data*/  cumulative mass function A (CMFA[1]) = normalizedhistogram A[1];  For (i=2 to Length of Histogram A)  { CMFA[i] =CMFA[i−1]+ normalized histogram A[i];  }

The same process is performed for a data set of a second condition(i.e., a second error rate). In particular, a histogram of relativemetrics corresponding to a second condition is calculated (block 820).The second condition is a second data set (i.e., series of values) thatwhen processed yield a second error rate. The histogram corresponding tothe second condition (block 820) is normalized (block 825). Thisnormalization may be done in accordance with the following pseudocode:

/* normalize histogram B */ For (i=1 to Length of Histogram B) {normalized histogram B[i] = (histogram B[i])/(sum of all elements ofhistogram B) }Histogram B corresponds to the histogram of the second condition. Thecumulative mass of the histogram of the second condition is calculated(block 827). This calculation may be done in accordance with thefollowing pseudocode:

/*compute a normalized cumulative mass function for the receivedhistogram data*/  cumulative mass function B (CMFB[1]) = normalizedhistogram B[1];  For (i=2 to Length of Histogram B)  { CMFB[i] =CMFB[i−1]+ normalized histogram B[i];  }

Bias values for converting between the first condition and the secondcondition are sequentially determined (block 830). These bias values maybe used to predictably degrade data processing operation as discussedabove in relation to FIG. 4, and in particular are the bias values usedin block 435 of FIG. 4. In some embodiments, determining the bias valuesis done in accordance with the following pseudocode:

/*compute bias values using the calculated cumulative mass function */Length of Biasing Look Up Table (LUT Length) = Length of Histogram A;Bias Value (LUT[ ]) = Array[LUT Length]; k = 1; For (i=1 to LUT Length){ if (i==1) { LUT[i] = 1 } Else { LUT[i] = LUT[i−1] } For (j=LUT[i] toLength of Histogram A) { If (abs_value(CMFA[i]− CMFA[LUT[i]])>abs_value(CMFA[i]− CMFA[j]) { LUT[i]=j } } While (k<=LUT[i]) { UpdatedCumulative Mass Function (UCMFA[ ]) = CMFA[i]; k = k+1 } }The calculated bias values (LUT[ ]) are then stored to a biasing look uptable from which they may be accessed for use in biasing (block 835).

Turning to FIG. 9, a histogram based bias value calculation circuit 900is shown in accordance with various embodiments of the presentinvention. Histogram based bias value calculation circuit 900 includes ahistogram generation circuit 905. Histogram generation circuit 905receives soft decision data from detected output 137, and generates ahistogram A 907 covering each value of the soft decision data for onedata set, and a histogram B 909 covering each value of the soft decisiondata for another data set. Histogram A 907 and histogram B 909 areprovided to a histogram normalization circuit 910 that normalizes bothof the received histograms to yield a normalized histogram A 917 and anormalized histogram B 919. In some embodiments, the normalizedhistograms are calculated in accordance with the following pseudocode:

/* normalize histogram A and histogram B */ For (i=1 to Length ofHistogram A) { normalized histogram A[i] = (histogram A[i])/(sum of allelements of histogram A); normalized histogram B[i] = (histogramB)[i]/(sum of all elements of histogram B) }

Normalized histogram A 917 and a normalized histogram B 919 are providedto a cumulative mass calculation circuit 920 that calculates acumulative mass A 927 of normalized histogram A 917, and a cumulativemass B 929 of normalized histogram B 919. In some embodiments, thecumulative mass calculations are performed in accordance with thefollowing pseudocode:

/* compute a normalized cumulative mass function for the receivedhistogram data*/ cumulative mass function A (CMFA[1]) = normalizedhistogram A[1]; cumulative mass function B (CMFB[1]) = normalizedhistogram B[1];  For (i=2 to Length of Histogram A) { CMFA[i] =CMFA[i−1]+ normalized histogram A[i]; CMFB[i] = CMFB[i−1]+ normalizedhistogram B[i] }

Cumulative mass A 927 and Cumulative mass B 929 are provided to a biasvalue calculation circuit 930 that calculates bias values 182. In someembodiments, the bias values are calculated in accordance with thefollowing pseudocode:

/* compute bias values 182 using the calculated cumulative mass function*/ Length of Biasing Look Up Table (LUT Length) = Length of Histogram A;Bias Value 182 (LUT[ ]) = Array[LUT Length]; k = 1; For (i=1 to LUTLength) { if (i==1) { LUT[i] = 1 } Else { LUT[i] = LUT[i−1] } For(j=LUT[i] to Length of Histogram A) { If (abs_value(CMFA[i]−CMFA[LUT[i]])> abs_value(CMFA[i]− CMFA[j]) { LUT[i]=j } } While(k<=LUT[i]) { Updated Cumulative Mass Function (UCMFA[ ]) = CMFA[i]; k =k+1 } }

It should be noted that the various blocks discussed in the aboveapplication may be implemented in integrated circuits along with otherfunctionality. Such integrated circuits may include all of the functionsof a given block, system or circuit, or only a subset of the block,system or circuit. Further, elements of the blocks, systems or circuitsmay be implemented across multiple integrated circuits. Such integratedcircuits may be any type of integrated circuit known in the artincluding, but are not limited to, a monolithic integrated circuit, aflip chip integrated circuit, a multichip module integrated circuit,and/or a mixed signal integrated circuit. It should also be noted thatvarious functions of the blocks, systems or circuits discussed hereinmay be implemented in either software or firmware. In some such cases,the entire system, block or circuit may be implemented using itssoftware or firmware equivalent. In other cases, the one part of a givensystem, block or circuit may be implemented in software or firmware,while other parts are implemented in hardware.

In conclusion, the invention provides novel systems, devices, methodsand arrangements for data processing. While detailed descriptions of oneor more embodiments of the invention have been given above, variousalternatives, modifications, and equivalents will be apparent to thoseskilled in the art without varying from the spirit of the invention.Therefore, the above description should not be taken as limiting thescope of the invention, which is defined by the appended claims.

1. A data processing circuit, the circuit comprising: a data detectorcircuit operable to apply a data detection algorithm to a series ofsymbols to yield a detected output, wherein the detected output includesa series of soft decision data corresponding to non-binary symbols; abiasing circuit operable apply a bias to each of the series of softdecision data to yield a series of biased soft decision data; and a datadecoder circuit operable to apply a data decoding algorithm to theseries of biased soft decision data corresponding to the non-binarysymbols.
 2. The data processing circuit of claim 1, wherein the datadetection algorithm is selected from a group consisting of: a non-binarymaximum a posteriori data detection algorithm and a non-binary Viterbidata detection algorithm.
 3. The data processing circuit of claim 1,wherein the data decoding algorithm is a non-binary low density paritycheck decoding algorithm.
 4. The data processing circuit of claim 1,wherein applying a bias to each of the series of soft decision dataincludes: converting the series of soft decision data to a correspondingseries of relative metrics; biasing a subset of the relative metrics toyield a series of biased relative metrics; and converting the series ofbiased relative metrics to a corresponding series of biased softdecision data.
 5. The data processing circuit of claim 4, whereinbiasing a subset of the relative metrics to yield a series of biasedrelative metrics includes replacing a relative metric with a biasedrelative metric accessed from a look up table.
 6. The data processingcircuit of claim 4, wherein the series of soft decision data is a seriesof non-binary soft decision data indicating a likelihood that respectivesymbol values are correct.
 7. The data processing circuit of claim 5,wherein the series of biased soft decision data is a series of biasednon-binary soft decision data indicating a likelihood that respectivesymbol values are correct, wherein the series of biased non-binary softdecision data indicate a lower likelihood than corresponding instancesof the series of non-biased soft decision data.
 8. The data processingcircuit of claim 7, the circuit further comprising: a bias value memory,wherein the bias value memory is operable to receive an address from thebiasing circuit and to return the bias value corresponding to theaddress, and where the bias value is used as a biased relative metric toreplace the corresponding relative metric.
 9. The data processingcircuit of claim 8, wherein the address is derived from one of theseries of relative metrics.
 10. The data processing circuit of claim 4,wherein the series of soft decision data includes four values (M₀₀, M₀₁,M₁₀ and M₁₁) corresponding to respective values of the non-binarysymbols, and wherein converting the series of soft decision data to acorresponding series of relative metrics (RM₀₀, RM₀₁, RM₁₀, and RM₁₁) isdone in accordance with the following equations:RM₀₀ =M ₀₀−Reference Metric,RM₀₁ =M ₀₁−Reference Metric,RM₁₀ =M ₁₀−Reference Metric, andRM₁₁ =M ₁₁−Reference Metric, wherein the Reference Metric is the metricvalue corresponding to an expected symbol value.
 11. The data processingcircuit of claim 4, wherein the series of biased soft decision dataincludes four values (M₀₀, M₀₁, M₁₀, and M₁₁) corresponding torespective values of the non-binary symbols, and wherein converting theseries of biased relative metrics to a corresponding series of biasedsoft decision data (UNBM₀₀, UNBM₀₁, UNBM₁₀, and UNBM₁₁) is done inaccordance with the following equations:UNBM₀₀=BRM₀₀−Minimum Relative Metric,UNBM₀₁=BRM₀₁−Minimum Relative Metric,UNBM₁₀=BRM₁₀−Minimum Relative Metric, andUNBM₁₁=BRM₁₁−Minimum Relative Metric, wherein the Minimum RelativeMetric is the biased relative metric (BRM₀₀, BRM₀₁, BRM₁₀, and BRM₁₁)that has the lowest value.
 12. The data processing circuit of claim 1,wherein the data processing circuit is implemented in an integratedcircuit.
 13. The data processing circuit of claim 1, wherein the dataprocessing circuit is implemented in a storage device.
 14. A method forpredictably increasing an error rate of a data processing circuit, themethod comprising: receiving a data set, wherein the data set includes aseries of symbols; applying a data detection algorithm by a datadetector circuit to a series of symbols to yield a detected output,wherein the detected output includes a series of soft decision datacorresponding to non-binary symbols; biasing each of the series of softdecision data to yield a series of biased soft decision data; andapplying a data decoding algorithm by a data decoder circuit to theseries of biased soft decision data corresponding to the non-binarysymbols.
 15. The method of claim 14, wherein biasing each of the seriesof soft decision data to yield the series of biased soft decision datacomprises: converting the series of soft decision data to acorresponding series of relative metrics; biasing a subset of therelative metrics to yield a series of biased relative metrics; andconverting the series of biased relative metrics to a correspondingseries of biased soft decision data.
 16. The method of claim 15, whereinthe series of biased soft decision data is a series of biased non-binarysoft decision data indicating a likelihood that respective symbol valuesare correct, and wherein the series of biased non-binary soft decisiondata indicate a lower likelihood than corresponding instances of theseries of non-biased soft decision data.
 17. The method of claim 15,wherein biasing the subset of the relative metrics to yield the seriesof biased relative metrics includes multiplying each of the subset ofthe relative metrics by a bias value.
 18. The method of claim 17,wherein the method further comprises: accessing a bias value memory toobtain the bias value.
 19. A data storage device, the storage devicecomprising: a storage medium; a sensor device disposed in relation tothe storage medium and operable to sense information from the storagemedium and to provide a corresponding analog signal; an analog todigital converter circuit operable to convert a derivative of the analogsignal to a series of digital samples; an equalizer circuit operable toperform an equalization on the series of digital samples to yield anequalized output, wherein the equalized output includes a series ofsymbols; a data detector circuit operable to apply a data detectionalgorithm to the series of symbols to yield a detected output, whereinthe detected output includes a series of soft decision datacorresponding to non-binary symbols; a biasing circuit operable apply abias to each of the series of soft decision data to yield a series ofbiased soft decision data; and a data decoder circuit operable to applya data decoding algorithm to the series of biased soft decision datacorresponding to the non-binary symbols.
 20. The data storage device ofclaim 19, wherein applying a bias to each of the series of soft decisiondata includes: converting the series of soft decision data to acorresponding series of relative metrics; biasing a subset of therelative metrics to yield a series of biased relative metrics; andconverting the series of biased relative metrics to a correspondingseries of biased soft decision data.